This invention relates to multilayer polyolefin films of the type suitable for packaging applications involving heat-sealing.
Multilayer polyolefin films incorporate a base or a substrate layer of a stereoregular crystalline thermoplastic polymer and one or more surface plies which can be formed on one or both sides of the base layer. Isotactic polypropylene is one of a number of crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. Various stereospecific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers from various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C3+alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g. styrene or vinyl chloride, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g. isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.
Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through dies and subject to biaxial orientation by stressing the film in both a longitudinal direction (referred to as the machine direction) and in a transverse or lateral direction sometimes referred to as the xe2x80x9ctenterxe2x80x9d direction. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: 
Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene as shown by Formula (2) is described as follows: 
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the polymer chain lie on alternate sides of the plane of the polymer. Using the Fisher projection formula, the structure of syndiotactic polypropylene can be shown as follows: 
Syndiotacticity can be characterized in terms of the syndiotactic pentad rrrr in which each xe2x80x9crxe2x80x9d represents a racernic dyad. Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are essentially insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
For many applications the preferred polymer configuration will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403 to Ewen. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes that may be characterized by the following formula:
Rxe2x80x3(C5(Rxe2x80x2)4)2 HfQpxe2x80x83xe2x80x83(4)
In Formula (4), (C5 (Rxe2x80x2)4) is a cyclopentadienyl or substituted cyclopentadienyl group, Rxe2x80x2 is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and Rxe2x80x3 is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
Metallocene catalysts, such as those described above, can be used either as so-called xe2x80x9cneutral metallocenesxe2x80x9d in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called xe2x80x9ccationic metallocenesxe2x80x9d which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures that are joined to a positively-charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components. Traditional supported catalysts are the so-called xe2x80x9cconventionalxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Mayr et al. A supported catalyst component, as disclosed in the Mayr ""718 patent, includes titanium tetrachloride supported on an xe2x80x9cactivexe2x80x9d anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Mayr ""718 is employed in conjunction with a co-catalyst such and an alkylaluminum compound, for example, triethylaluminum (TEAL). The Mayr ""717 patent discloses a similar compound that may also incorporate an electron donor compound which may take the form of various amines, phosphenes, esters, aldehydes, and alcohols.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welbom ""432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn ""561 patent discloses a heterogeneous catalyst that is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a xe2x80x9cconventionalxe2x80x9d supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Sharishoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al and U.S. Pat. No. 5,444,134 to Matsumoto.
The polymers normally employed in the preparation of biaxially-oriented polypropylene films are usually those prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in the aforementioned patents to Mayr et al. Thus, U.S. Pat. No. 5,573,723 to Peiffer et al discloses a process for producing biaxially-oriented polypropylene film having a base layer formed of an isotactic polypropylene homopolymer or propylene ethylene co-polymers. Other co-polymers of propylene and alpha-olefins having from 4-8 carbon atoms also may be employed in the Peiffer process. Thus, the base layer may take the form of a mixture of isotactic polypropylene or ethylene propylene copolymers with resin polymers such as styrene homopolymers having a softening point of about 130-180xc2x0 C. The surface layer or layers may likewise take the form of a propylene homopolymer or copolymer of the same type employed in the base layer.
Processes for the preparation of biaxially-oriented polypropylene films employing polymers produced by the use of isospecific metallocenes involving di- or tri- substituted indenyl groups are disclosed in Canadian Patent Application No. 2,178,104. Four isotactic polymers disclosed there are based upon the polymerization of propylene in the presence of heavily substituted bis(indenyl) ligand structures. In each case, the metallocene used was a silicon-bridged di-or tri- substituted bis(indenyl) zirconium dichloride. More specifically, the metallocene catalysts used are identified in the aforementioned Canadian patent as rac-dimethylsilanediethyl bis(2-methyl-4,6 diisopropyl-1 indenyl) zirconium dichloride, 2 rac-dimethylsilanediethyl bis(2-methyl4,5-benzo-1-indenyl) zirconium dichloride, 3 rac-dimethylsilanediethyl bis(2-methyl-4-phenyl-1-indenyl) zirconium dichloride, and 4 rac-dimethylsilanediethyl bis(2-ethyl4phenyl-1-indenyl) zirconium dichloride. The various polymers produced by these metallocenes catalysts are characterized in terms of molecular weight, molecular weight distribution, melting point, meltflow index, mean isotactic block length, and isotactic index as defined in terms of mm triads. The polymers produced had isotactic indices, as thus defined, of about 97-98% as contrasted with an isotactic index of 93% for a commercial polypropylene compared with a conventional Ziegler-Natta catalyst and molecular weight distributions ranging from about 2.0 to 3.0 as contrasted with a molecular weight distribution of 4.5 for the polypropylene produced by the conventional Ziegler-Natta catalyst. Similarly, as in the case of the aforementioned patent to Peiffer et al, the Canadian ""104 application discloses multilayer films in which the base ply and one or two top plies can be formed of the same or different propylene polymers including propylene homopolymers or copolymers or terpolymers. Where a propylene homopolymer is employed in the top ply, it is described as having a melting point of at least 140xc2x0 C. Similarly, as in the case of the aforementioned patent to Peiffer et al, the Canadian ""104 application discloses multilayer films in which the base ply and one or two top plies can be formed of the same or different propylene polymers including propylene homopolymers or copolymers or terpolymers. Where a propylene homopolymer is employed in the top ply, it is described as having a melting point of at least 140xc2x0 C. and a melt flow index of 1 to 20 grams/10 minutes. In the Canadian ""104 application a typical film structure, the base ply is characterized as providing at least 40% and typically 50-98% of the total film thickness with the outer ply or plies supplying the remainder of the film thickness. Specific overall film thicknesses disclosed in the Canadian ""104 application range from 4 to 100 microns and more specifically 6 to 30 microns with the base ply specifically ranging from 1.5 to 10 microns and the outer plies from 0.4 to 1.5 microns.
In accordance with the present invention, there is provided a multilayer polyolefin film of the type suitable for packaging application in which heat seals are formed. The multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer. The surface layer is formed of a syndiotactic propylene polymer which is effective to produce a heat seal with itself at a sealing temperature of less than 110xc2x0 C. The surface layer has a thickness which is less than the thickness of the substrate layer. Preferably, the substrate layer has an average thickness within the range of 5-150 microns, and the surface layer has a thickness which is no more than one-half the thickness of the substrate layer, preferably no more than ⅓ of the thickness of the substrate layer and having a thickness within the range of 0.3-50 microns. Preferably, the heat-seal layer is formed of syndiotactic polypropylene polymerized in the presence of a syndiospecific metallocene catalyst and having a melt flow index of less than 2 grams/10 minutes. Preferably, the multilayer film is a biaxially-oriented film.
In a further aspect of the invention, there is provided a process for the production of a multilayer film incorporating a substrate layer and a heat-sealable surface layer. In carrying out the invention, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer is employed comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer. The propylene polymer is extruded to form a surface layer that is bonded to the interface of the substrate layer at a temperature within the range of 150-260xc2x0 C.